High-risk drug injection behavior (e.g., injecting with shared syringes, cookers, cotton, and/or water, or splitting drugs with shared syringes) is a significant route of transmission for human immunodeficiency virus (HIV) among IDUs. Though reductions in HIV risk behavior among IDUs have been observed in recent years, residual injection risk behavior persists. For example, from 2000 to 2003, 38% of California syringe exchange program (SEP) participants reported syringe sharing and up to 62% reported paraphernalia sharing. Significant gender differences exist in HIV risk among IDUs. Among women, injection-related risk is often associated with the social and environmental context in which they use drugs, whereas this influence appears to be less significant among men. Several individual-level, cognitive behavioral theories have been used to explain risky injection practices, employing constructs such as perceived susceptibility, perceived severity, self-efficacy, response efficacy, social norms, and HIV knowledge. To date, theoretically-based studies have had mixed success in explaining injection risk behavior using these constructs. Few studies have examined the association between perceived costs or barriers to safer injection and injection risk, and even fewer have examined gender differences in the explanatory power of these theoretical models. These omissions may partially explain the inconsistent results reported by existing studies. In order to more fully understand the persistence of injection risk behavior, the proposed study will test a theoretical model that integrates both individual-level cognitive and environmental- level factors in the form of perceived costs of safer injection. The model incorporates constructs from several cognitive theoretical models, with a focus on the moderating influence of perceived costs and gender. The proposed study will collect data from 20 semi-structured, qualitative interviews and 200 cross- sectional, quantitative interviews with IDUs recruited from a Los Angeles, California SEP. The sample will be ethnically diverse and 33% female. The study has four specific aims: 1) Identify the perceived costs associated with safer injection behavior via qualitative interviews and develop a measure of perceived costs that will be used in a quantitative survey;2) Assess the relationship between HIV-related cognitive factors and injection risk behavior;3) Use Structural Equation Modeling (SEM) to explore the moderating influence of perceived costs on the association between other cognitive variables and injection risk behavior;and 4) Use SEM to explore gender differences in perceived costs and in the strength of associations between cognitive and behavioral variables. Findings from the proposed study could be used to design interventions to mitigate the social and environmental costs of safer injection among IDUs, and specifically to help alleviate barriers to safer injection among female IDUs. If the costs of safer behavior can be minimized, IDUs may be more likely to act upon existing HIV cognitions to reduce their injection-related risk behavior. By identifying those factors that contribute to persistent injection-related risk for HIV among injection drug users, interventions designed to address those factors can be developed. Specifically, this study will identify the perceived costs of safer injection behavior, allowing for the development of HIV prevention interventions designed to mitigate the costs of safer behavior. Additionally, a detailed understanding of how these factors differ by gender will help in the design and implementation of tailored interventions that take into account the social and environmental context of both men and women, and those factors that contribute to elevated risk for HIV among women.